The invention relates generally to ultra-low abrasion slip rings and method and apparatus for burnishing a surface of a material.
There are many applications which require a smooth surface finish. One application requiring an ultra-low abrasion finish is a contact surface of a slip ring assembly. One of the problems associated with a slip ring assembly is that a contacting element such as a brush wears prematurely due to the roughness of the contact surface. The premature wear may be reduced by burnishing the contact surface.
Several devices are currently available to burnish a surface of a material. However, none of the devices thus far appear to be without problems. U.S. Pat. No. 3,606,708 to Plichta et al. discloses a burnishing apparatus for smoothing metal coated surfaces by successive rolling and burnishing operations. The apparatus performs rolling and burnishing operations using hard polished rollers and a flexible burnishing wheel. A workpiece is advanced through the rolling and burnishing stations by two opposed conveyor belts which grip the pieces therebetween with a portion to be rolled and burnished protruding to one side of the belts. However, due to the conveyor belt arrangement, it is difficult to selectively burnish an annular contact surface of a slip ring assembly without burnishing the entire surface of the slip ring assembly. In certain circumstances, the surface of the slip ring assembly may include various electrical components which should not or do not require burnishing.
U.S. Pat. No. 3,820,210 to Kalen discloses a burnishing tool which is more applicable to burnishing an annular contact surface. The burnishing tool has a head connected to a spindle which is rotatively driven. The head has an end face with balls, and a workpiece is burnished by driving and rotating the head and balls on the surface of the workpiece. The balls produce a circular burnished track having a width w. The burnishing operation may be spread to all points of the workpiece by advancing the head over the flat area of the workpiece. The workpiece is mounted on a lead screw-operated table and movement of the table during burnishing may form an eccentric burnishing path. However, it may be difficult to properly advance the workpiece to form a well defined annular burnished path when the width of the burnished path W is greater than the width w. This is particularly important for a slip ring assembly requiring tight processing tolerances due to the high density of electrical components on the surface of the assembly.
Thus, there remains a need for a burnishing apparatus that accurately and precisely forms ultra-low abrasions annular surfaces.
In accordance with the present invention, a burnishing apparatus burnishes a surface of a material using rolling elements. The burnishing apparatus is particularly suited for burnishing an annular path having a width greater than the track width of the rolling elements. Generally, in accordance with an exemplary illustrative embodiment of the present invention, the burnishing apparatus comprises a burnishing unit, a control unit, and a compressed air source unit.
The burnishing unit includes a burnishing assembly, a drive unit for rotating the burnishing assembly, a positioning/securing unit for properly aligning and securing the slip ring substrate onto the burnishing unit, and a pneumatic actuator for directing a predetermined force on the substrate such that the surface of the substrate is forced onto the rolling elements.
The burnishing assembly includes a burnishing disk, rolling elements such as burnishing balls, a pressure pad, and a top cover. The burnishing disk has a first and second annular groove on its surface. The first and second groove have a common center axis which is offset from the rotating axis of the burnishing disk. The first groove guides a first pair of burnishing balls, and the second groove guides a second pair of burnishing balls. The ball retainer is a disk-shaped element having a first and second pair of apertures for retaining the four burnishing balls. The substrate is positioned such that the surface with the slip ring faces the burnishing balls, and the substrate is secured to the burnishing unit by the positioning/securing unit. The pneumatic actuator directs the top cover onto the back side of the slip ring substrate and forces the surface with the slip ring onto the bearings. The drive unit includes a motor, and the motor is coupled to the burnishing disk.
The burnishing balls have a radius r, an effective rolling radius on the guide member r1, and an effective rolling radius on the surface of the substrate r2, and r=r2 and r1 less than r2. Since r1 less than r2, the angular traverse of the burnishing balls on the surface of the substrate xcfx86 greater than the angular traverse of the burnishing balls on the surface of the guide member. Due to the combined effects of the center axis of the grooves being offset from the rotating axis of the burnishing disk and r1 less than r2, a variable track is left on the surface of the substrate as the burnishing balls roll along the grooves and on the surface of the substrate. Since each groove has a pair of burnishing balls and each burnishing ball leaves a track width w, the entire surface of the slip ring is covered after the pair of burnishing balls complete N revolutions around the surface of the substrate. The various components of the burnishing assembly are configured so that the variable track repeats every N revolutions, wherein Nxc2x7w is xe2x89xa7W to burnish a ring of width W.
The control unit is coupled to the burnishing unit, and an operator may set the various processing parameters such as the load directed on the substrate by the pneumatic actuator, the rotation rate of the burnishing disk, and the number of revolutions N. The compressed air source unit supplies compressed air for operating the pneumatic actuator.
Other objects, features, and advantages of the present invention will become apparent from a consideration of the following detailed description.